Birds of a feather may flock together -- but they don't sing together if they live in the city. It turns out birds that live in the city sing at higher frequencies, louder, and more often than their rural friends to outmatch the noise pollution of cars and people. The din of city life is creating new divisions between bird species; Researchers like Elizabeth Derryberry are finding those high-pitched tenors of the bird world aren't quite as attractive to mates as the lower Barry Whites of the country.